The present invention relates to an actuator for a chuck. More particularly this invention concerns a hydraulic actuator for a lathe chuck.
A standard chuck assembly has an actuator cylinder centered on and rotatable about an axis but substantially nondisplaceable relative to the axis and a chuck housing spaced axially from the cylinder. A tubular connecting shaft extending along the axis is connected fixedly between the cylinder and the chuck housing so that the cylinder, connecting shaft, and chuck housing rotate jointly about the axis but do not move along the axis. A chuck piston axially displaceable in the chuck housing has an inner end turned axially toward the actuator cylinder, and an actuator piston axially is displaceable in the cylinder. The chuck piston and actuator piston are axially interconnected by a shaft assembly comprising an actuator shaft fixed to the actuator piston and having an outer end and a chuck shaft having an inner end connected to the actuator-shaft outer end and an outer end connected to the chuck-piston inner end. The chuck has radially displaceable jaws operated by its piston so that a workpiece can be gripped between these jaws and rotated about the axis for machining.
It is important to know how tightly the jaws are squeezing the workpiece. This force is directly related to the axial force the shaft assembly transmits to the chuck piston which is equal to the difference between the pressures in inner and outer compartments flanking the actuator piston. Thus it is relatively simple as described in German patent document 195 39 195 to provide pressure sensors in these compartments or in the hydraulic lines connected to them. The two pressures are read and the difference taken to determine the clamping force.
Such a system is relatively inaccurate. According to the Gaussian theory of error multiplication the measurement errors of the two read-out values are determined by the square root of the sum of the squares of the absolute inaccuracies of the individual measurements. Thus it is clear that the force determined in this manner can be relatively inaccurate.
It is therefore an object of the present invention to provide an improved chuck assembly.
Another object is the provision of such an improved chuck assembly which overcomes the above-given disadvantages, that is which allows the user to very accurately determine the clamping force.
A chuck assembly has according to the invention an actuator cylinder centered on and rotatable about an axis but substantially nondisplaceable relative to the axis, a chuck housing spaced axially from the cylinder, and a tubular connecting shaft extending along the axis and connected fixedly between the cylinder and the chuck housing so that the cylinder, connecting shaft, and chuck housing rotate jointly about the axis but do not move along the axis. A chuck piston axially displaceable in the chuck housing has an inner end turned axially toward the actuator cylinder. An actuator piston is similarly axially displaceable in the cylinder. The two pistons are connected together by a shaft assembly that has an actuator shaft fixed to the actuator piston and having an outer end and a chuck shaft having an inner end connected to the actuator-shaft outer end and an outer end connected to the chuck-piston inner end. According to the invention a force sensor provided between one of the inner ends and the respective outer end detects axial force along the shaft assembly.
Thus in accordance with the invention the force is measured right in the shaft assembly transmitting the force from the actuator to the chuck. Since the losses between this shaft and the jaws are inconsequential, the force thus detected very accurately represents the clamping force. There are no cumulative errors; instead a single force reading is taken.
The sensor according to the invention can be between the actuator-shaft outer end and the chuck-shaft inner end. In this situation the chuck-shaft includes an extension piece forming its inner end. Such an installation is particularly suitable as a retrofit. The use of an intermediate piece reduces difficulties in calibrating the assembly.
Alternately the sensor is between the chuck-shaft outer end and the chuck-piston inner end. Putting it right at the chuck ensures that its output will most accurately reflect clamping force.
The chuck assembly according to the invention further has a transmitter axially and rotationally fixed to the cylinder, connecting shaft, and chuck housing, a connecting line between the transmitter and the sensor for outputting axial force detected by the sensor, and a receiver juxtaposed with the transmitter but nonrotatable about the axis for receiving the transmitted force. The transmitter can be mounted on the actuator shaft. In this case the chuck assembly further comprises a distributor body axially fixed but rotatable on an inner end of the actuator shaft and carrying the receiver. The existing fluid bores in the distributor body can be used to run the output lines, so that the system of this invention does not greatly complicate manufacture of the chuck assembly. The transmitter can also be mounted on the cylinder or on the chuck housing.
The sensor in accordance with the invention is piezoelectric or a strain gauge. It can also be a pressure compartment associated with such a pressure detector.